linux-kernel - RE: [PATCH] kvm: handle last_boosted

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Date:	Thu, 28 Jun 2012 23:55:54 +0100
From:	"Vinod, Chegu" <chegu_vinod@...com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Andrew Jones <drjones@...hat.com>
CC:	Rik van Riel <riel@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
	Gleb Natapov <gleb@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...hat.com>
Subject: RE: [PATCH] kvm: handle last_boosted_vcpu = 0 case

Hello,

I am just catching up on this email thread... 

Perhaps one of you may be able to help answer this query.. preferably along with some data.  [BTW, I do understand the basic intent behind PLE in a typical [sweet spot] use case where there is over subscription etc. and the need to optimize the PLE handler in the host etc. ]

In a use case where the host has fewer but much larger guests (say 40VCPUs and higher) and there is no over subscription (i.e. # of vcpus across guests <= physical cpus in the host  and perhaps each guest has their vcpu's pinned to specific physical cpus for other reasons), I would like to understand if/how  the PLE really helps ?  For these use cases would it be ok to turn PLE off (ple_gap=0) since is no real need to take an exit and find some other VCPU to yield to ? 

Thanks
Vinod

-----Original Message-----
From: Raghavendra K T [mailto:raghavendra.kt@...ux.vnet.ibm.com] 
Sent: Thursday, June 28, 2012 9:22 AM
To: Andrew Jones
Cc: Rik van Riel; Marcelo Tosatti; Srikar; Srivatsa Vaddagiri; Peter Zijlstra; Nikunj A. Dadhania; KVM; LKML; Gleb Natapov; Vinod, Chegu; Jeremy Fitzhardinge; Avi Kivity; Ingo Molnar
Subject: Re: [PATCH] kvm: handle last_boosted_vcpu = 0 case

On 06/28/2012 09:30 PM, Andrew Jones wrote:
>
>
> ----- Original Message -----
>> In summary, current PV has huge benefit on non-PLE machine.
>>
>> On PLE machine, the results become very sensitive to load, type of 
>> workload and SPIN_THRESHOLD. Also PLE interference has significant 
>> effect on them. But still it has slight edge over non PV.
>>
>
> Hi Raghu,
>
> sorry for my slow response. I'm on vacation right now (until the 9th 
> of July) and I have limited access to mail.

Ok. Happy Vacation :)

Also, thanks for
> continuing the benchmarking. Question, when you compare PLE vs.
> non-PLE, are you using different machines (one with and one without), 
> or are you disabling its use by loading the kvm module with the 
> ple_gap=0 modparam as I did?

Yes, I am doing the same when I say with PLE disabled and comparing the benchmarks (i.e loading kvm module with ple_gap=0).

But older non-PLE results were on a different machine altogether. (I had limited access to PLE machine).